Immediately after infection T4 bacteriophage modifies the valyl- tRNA synthetase of Escherichia coli by adding a small peptide factor Tau to the host enzyme. As a result of Tau binding the sedimentation velocity and electrophoretic mobility of the modified enzyme are increased in the presence tRNA. Though the modified enzyme per se is intrinsically more stable than the normal enzyme to denaturation by heat and 4M urea treatment, tRNA additionally stabilizes the molecule. Materials have been developed for an immunochemical assay for the Tau peptide. The assay will be used: to study the regulation of Tau synthesis, to analyze T4 phage mutants impaired in valyl-tRNA synthetase modification, to analyze other cell components for Tau association and to see whether Tau is associated with the mature T4 virus. An attempt will also be made to identify the particular tRNA species that is responsible for specific effects on the virally modified valyl-tRNA synthetase.